As of last month, Metropolitan Washington Airports Authority officials were hopeful that construction of the first phase of the Silver Line would reach substantial completion by the end of 2013.

However, as the new year begins, final testing of the rail line is ongoing, according to spokeswoman Marcia McAllister.

Construction is about 99.8 percent done, she said, but there is still more testing to be done.

At a December meeting of the MWAA Board of Directors, Dulles Rail Project Executive Director Pat Nowakowski said that the biggest hurdle was addressing issues with the automated train control system.

Once the issues discovered during testing are resolved, the project contractor must do a simulated service test, which can only be conducted on the overnight hours between Saturday night and Sunday morning, due to the effect on Orange Line service, Nowakowski told the board last month. Therefore, every missed opportunity to conduct that test adds another week to the schedule.

When contractor Dulles Transit Partners believes it has achieved substantial completion, MWAA has 15 days to review and accept the completed project. The authority will then begin the process of transferring the new line into the Metro system.

The Washington Metropolitan Area Transit Authority will then need to conduct its own testing, employee training and safety drills before service begins. Metro will be responsible for setting the date for the start of service on the rail line, which agency officials said they will not do until the Silver Line is transferred.